PROJECT SUMMARY B cell responses are essential for clearance of viral infections. The mechanisms by which chronic viruses subvert B cell immunity are poorly understood. Chikungunya virus (CHIKV) is a mosquito-transmitted RNA alphavirus that causes explosive epidemics of debilitating polyarthralgia. Joint swelling, joint stiffness, and tenosynovitis can last for months to years and are associated with persistent CHIKV infection. The mechanistic basis for how CHIKV evades B cell immunity to establish chronic infection is unknown. We found that an attenuated CHIKV strain, but not the parental CHIKV strain (which differs by 5 amino acids), is cleared from joints of WT mice. Persistent infection of the attenuated strain was restored in mice unable to produce virus-specific antibody (Ab). Mapping studies revealed that a single arginine or glycine residue at position 82 in the CHIKV E2 glycoprotein dictates viral clearance or persistence, respectively. In comparison with acutely cleared CHIKV, persistent CHIKV infection was associated with altered B cell responses, a failure to develop germinal centers in the draining lymph node (DLN), poorer quality virus-specific neutralizing Abs, and distinct viral localization and inflammatory responses in lymphoid tissue. Remarkably, depletion of inflammatory myeloid cells improved B cell responses to pathogenic CHIKV. The goal of this highly interactive project between the Morrison and Diamond laboratories is to define mechanistically how pathogenic CHIKV strains evade B cell immunity and the contribution of inflammatory monocytes to this process. In Specific Aim 1, the mechanisms by which a specific CHIKV E2-82 residue dictates inflammatory responses in the DLN will be determined. Viruses differing only at E2-82, and new mice with genetic deficiencies in glycosaminoglycans (GAGs), will be used to determine how E2-GAG interactions dictate CHIKV localization in the DLN. LN macrophage depletion and single-cell RNAseq will define the role of LN myeloid cells in inflammatory and B cell responses to persistent and acutely cleared CHIKV. In Specific Aim 2, how type I IFN signaling modulates B cell responses during acutely cleared and persistent CHIKV infection will be defined. Type I IFN reporter mice, flow cytometry, and IHC will be used to define the cells that produce type I IFN in lymphoid tissue. Ab-mediated blockade of IFNAR1 or distinct type I IFN subtypes (pan-? versus ?), and mice with B cell-specific defects in type I IFN signaling will be used to determine the effects of type I IFN on B cell, Th, and Tfh cell responses. In Specific Aim 3, how inflammatory myeloid cells antagonize B cell responses during persistent CHIKV infection will be elucidated. The impact of inflammatory myeloid cells on the avidity, neutralization capacity, and epitope repertoire of the polyclonal anti- CHIKV Ab response will be defined. Using new Nos2F/F or Nox2F/F mice, and a panel of Cre driver strains, we will determine mechanisms by which specific subsets of myeloid cells inhibit optimal B cell responses. This work will elucidate new mechanisms by which viruses subvert B cell immunity and establish persistence. The knowledge gained may aid the development of vaccines or therapies against CHIKV or other chronic viral infections.